Electrodialysis is a well known art (see U.S. Pat. Nos. 4,325,792; 4,439,293; 4,636,288; 4,080,270, the disclosures of which are incorporated by reference). Electrodialysis is the transport of ions through ion permeable membranes as a result of an electrical driving force. The process is commonly carried out in an electrochemical cell having a catholyte compartment containing a cathode and a catholyte and an anolyte compartment containing an anode and an anolyte, the catholyte and anolyte compartments being separated by ion permeable membranes. There is always in every electrodialytic process some small degree of reverse migration of cations through anion permeable membranes and/or anions through cation permeable membranes. Oxidations occur at the cell anode and reductions at the cell cathode.
The electrodialytic processes as disclosed in my U.S. Pat. Nos. 4,325,792; 4,439,293; 4,636,288 provide a method for electrotransport of multivalent metal cations through cation permeable membranes to effect the electrodialytic conversion of multivalent metal salts with and without admixture with monovalent metal salts. These processes are now used broadly in the industry, with satisfactory results. There are, however, uses where the anion required to form a soluble salt with a multivalent metal cation to facilitate electrotransport of the multivalent cation is unstable at the cell cathode or the anion reverse migrates to the feed electrolyte or anolyte and is undesirable or is converted to an undesirable gas at the cell anode. There are also mixtures of multivalent metal salts where two or more anions are required in the catholyte or electrolyte receiving the multivalent metal cations to effect electrotransport of the multivalent metal cations without fouling the ion transport of the cation permeable membrane. If there was an anion of one acid that would facilitate the electrotransport of all multivalent metal cations through cation permeable membranes and if this anion was stable in the electrodialytic process and reverse migration was minimal, all multivalent metal salts and mixtures could be electrodialytically converted to the acids or halogen of the salt anion and the hydroxides or insoluble salts of the multivalent metal cations in a simple, high capacity electrodialytic process.
Acids are used broadly in the chemical, electronics, mining, electroplating and metal finishing industries wherein the acids react with metals and other salts to form salts of multivalent cations and anions of the respective acids. Often, there are salts of several multivalent metal cations and different anions in solutions. For example, in chromic acid solutions used to electroplate chromium, the anodes of lead, tin and antimony dissolve to form salts, barium carbonate is used to adjust the concentration of sulfate catalyst, strontium is added regulate or form a limited solubility catalyst, magnesium silicon fluoride is added as a co-catalyst or inhibitor, chemicals enter the chromic acid solution on parts to be plated, parts are dissolved or etched into the solution and calcium and other salts are added with make-up water. To purify and restore the plating solution, all metal cations must be removed and all desirable anions left in the chromic acid solution. If chloride ions are used to facilitate the electrotransport of the multivalent metal cations, the chloride adversely affects electroplating if it reverse migrates it is converted to chlorine at the cell anode. If nitrate ions are used, they can be converted to ammonium cations or ammonia gas in the catholyte and would be undesirable if the nitrate ions reverse migrate into the chromic acid solutions. Based on available data, it would be surprising if there was an anion of one acid that would facilitate the electrotransport of all of the ionically mobile multivalent metal cations from the chromic acid solution through a cation permeable membrane. More surprising would be that barium salts can be electrodialytically converted by electrotransport of barium cations through a cation permeable membrane having sulfonic acid groups fixed to a polymer matrix into an electrolyte containing a soluble sulfonate anion.
It is an object of the instant invention to provide a simple, efficient, reliable high capacity electrodialytic process for the electrodialytic conversion of barium, lead, calcium, strontium and other multivalent metal salts and mixture of salts using a soluble anion of a sulfonic acid to facilitate electrotransport of the multivalent metal cations from an electrolyte through a cation permeable membrane.